Types of check valves
Forward through a hole pushes away a ball that was blocking the far
side of the hole, allowing flow to occur. The motion of the ball
compresses a spring.
Backward flow pushes a ball against a hole, preventing outward flow
through the hole. A spring expands to help hold the ball in place.
A ball check valve in the open position to allow forward flow and
closed position to block reverse flow
A ball check valve is a check valve in which the closing member,
the movable part to block the flow, is a spherical ball. In some
ball check valves, the ball is spring-loaded to help keep it shut.
For those designs without a spring, reverse flow is required to
move the ball toward the seat and create a seal. The interior
surface of the main seats of ball check valves are more or less
conically-tapered to guide the ball into the seat and form a
positive seal when stopping reverse flow.
Ball check valves are often very small, simple, and cheap. They are
commonly used in liquid or gel minipump dispenser spigots, spray
devices, some rubber bulbs for pumping air, etc., manual air pumps
and some other pumps, and refillable dispensing syringes. Although
the balls are most often made of metal, they can be made of other
materials; in some specialized cases out of highly durable or inert
materials, such as sapphire.
High pressure HPLC pumps and similar applications commonly use
small inlet and outlet ball check valves with both balls and seats
made of artificial ruby, for both hardness and chemical
resistance. After prolonged use, such check valves
can eventually wear out or the seat can develop a crack, requiring
Therefore, such valves are made to be replaceable, sometimes placed
in a small plastic body tightly-fitted inside a metal fitting which
can withstand high pressure and which is screwed into the pump
There are similar check valves where the disc is not a ball, but
some other shape, such as a poppet energized by a spring. Ball
check valves should not be confused with ball valves, which is a
different type of valve in which a ball acts as a controllable
rotor to stop or direct flow.
A diaphragm check valve uses a flexing rubber diaphragm positioned
to create a normally-closed valve. Pressure on the upstream side
must be greater than the pressure on the downstream side by a
certain amount, known as the pressure differential, for the check
valve to open allowing flow. Once positive pressure stops, the
diaphragm automatically flexes back to its original closed
A swing check valve or tilting disc check valve is a check valve in
which the disc, the movable part to block the flow, swings on a
hinge or trunnion, either onto the seat to block reverse flow or
off the seat to allow forward flow. The seat opening cross-section
may be perpendicular to the centerline between the two ports or at
Although swing check valves can come in various sizes, large check
valves are often swing check valves. The flapper valve in a
flush-toilet mechanism is an example of this type of valve. Tank
pressure holding it closed is overcome by manual lift of the
flapper. It then remains open until the tank drains and the flapper
falls due to gravity.
Another variation of this mechanism is the clapper valve, used in
applications such firefighting and fire life safety systems. A
hinged gate only remains open in the inflowing direction. The
clapper valve often also has a spring that keeps the gate shut when
there is no forward pressure. Another example is the backwater
valve (for sanitary drainage system) that protects against flooding
caused by return flow of sewage waters.
Such risk occurs most often in sanitary drainage systems connected
to combined sewerage systems and in rainwater drainage systems. It
may be caused by intense rainfall, thaw or flood.
A stop-check valve is a check valve with override control to stop
flow regardless of flow direction or pressure. In addition to
closing in response to backflow or insufficient forward pressure
(normal check-valve behavior), it can also be deliberately shut by
an external mechanism, thereby preventing any flow regardless of
A lift-check valve is a check valve in which the disc, sometimes
called a lift, can be lifted up off its seat by higher pressure of
inlet or upstream fluid to allow flow to the outlet or downstream
side. A guide keeps motion of the disc on a vertical line, so the
valve can later reseat properly. When the pressure is no longer